M3: A pager

Use your memory manager from M2 to write a simple VM fault handler that
supports on-demand memory mapping of an application and an
allocate-on-demand memory heap.

Current Implementation

The current implementation simply pre-maps in the pages for
the single binary. Any actual VM page faults will trigger an
assert() which halts the system, under the assumption something
has gone wrong. The executable itself is mapped in with untracked
frames (i.e. we leak them), and finally, the current sos
application malloc implementation uses a static
array.

A small malloc() intro

'malloc' is the standard library function to allocate memory in
C. In our system, malloc is provided by the musl libc
library. malloc manages memory from a bigger memory pool. In the
SOS code you are provided, malloc uses memory from a
pre-allocated array in the initial data section as the memory
pool. This pool is fixed in size, and malloc returns NULL when the pool
is exhausted. See the diagram below for an approximate picture of
the memory layout. The code musl uses to allocate memory from the
static region for SOS is in apps/sos/src/sys/sys_morecore.c

SOS applications use an independent implementation that
uses a pre-allocated memory pool in the application's data section
(as shown in the middle of the diagram). The code musl libc uses in
applications is in libs/libsos/src/sys_morecore.c. So
just for emphasis, there are two versions of the morecore code
in the system.

One of the tasks of this milestone is to leverage virtual
memory to create a dynamically allocated memory region for
malloc's use, usually termed the heap, as shown at the
bottom of the diagram. The dynamic region is allocated on-demand
via VM faults, and can be expanded in range dynamically by
requesting that SOS increase the brk point.

In this milestone, you will modify the application-level
morecore routines in libs/libsos/src/sys_morecore.c
to use your dynamically-allocated memory region. Again, be sure
you're modifying the morecore routines for sos applications, not
SOS's own morecore routines.

Malloc and musl libc peculiarities

The malloc implementation in musl libc (i.e. the
application C library) has two behaviours for implementing
memory allocation of the memory pool:

Musl expects a brk syscall that has similar
semantics to the Linux brksyscall. The
current implementation is sys_brk
in libs/libsos/src/sys_morecore.c and it returns
memory from a static array, as described above.

If malloc is called to allocate more than
112KiB, then musl libc does not allocate from
the heap or increase the heap via sys_brk,
but instead it uses mmap to create an large
anonymous mapping. The sample implementation of this is
the sys_mmap2 function
in libs/libsos/src/sys_morecore.c and it
currently steals memory from the top of the static
morecore region, and leaks memory on when it's released
with munmap

The Milestone

In this milestone you will:

Implement a page table to translate virtual memory
addresses to frame table entries. You need to consider
where you will keep track of cptrs to both levels of the
page table and to the frames themselves. Note: ARM uses a
16K level-one page directory and a 1K level-two page
table, i.e. 12-bits index the top level, and 8-bits index
the second level. You do not have to follow this split,
you have freedom to choose whatever split is convenient
for your implementation of SOS. However, you do need to
theoretically keep track of the cptrs to an in-kernel page
tables to be able to de-allocate them. So if SOS
applications have a 2GB address space, you potentially
need to keep track of 2^11 cptrs.

Using our code as a guide only, create your own
version of the map_page(). Your version
(say sos_map_page()) will need to populate
and use your data structures to keep track of address
spaces, virtual addresses, frame physical addresses and
cptrs. Note: The existing map_page() is used
internally and thus needs to continue to work as before,
so be careful if modifying it.

Design your application-level address space layout and
permissions - including the stack, heap and other
sections. You may wish to consider a region-like
abstraction like OS/161.

Change the current application-level malloc
implementation to use virtual memory by modifying
the sys_brk() to use the heap memory region,
thus removing the need for a static array (just allow the
application to fault in memory within the heap region), as
shown at the bottom of the above diagram.

At this point, you do not need to support
the brk system call to expand the
region. You can choose a fixed (large-ish) virtual
memory range. You can add a brk()
system call in a later milestone.

You are not required to support
the mmap syscall for anonymous memory
in this project. We don't expect you to support
applications calling malloc for sizes
over 112KiB (you can gracefully fail
those requests in the library). Note: there is a
small bonus available for those who do implement
mmap/munmap.

Modify the bootstrap ELF-loading code to use your
mapping functions, not the frame table
directly.

Design alternatives

Probably the main thing that you should consider here is the
layout of your processes address space. Some things you will want
to consider is where you place various parts of memory such as the
stack, heap and code segments. You may also have some other
regions in your process address space, one of these is the IPC
Buffer.

You should also think about if you want to make different
ranges of the address space have different permissions, eg: you
may want to make code read-only to prevent bugs, or have a guard
page at the end of your stack to prevent overflow.

While not needed for this milestone, you should think about
what book keeping is required to delete an address space and free
all the resources associated with it.

Assessment

Demonstration

The main demonstration here will be to show a user process
running with a high stack pointer (> 0x20000000). You should also
demonstrate a user process using malloc() from a heap.